There is known a shaft drive type motorcycle in which a drive shaft is provided on the lower side of a pivot shaft, and a driving force of an engine is transmitted to a rear wheel by the drive shaft (see, for example, Patent Document 1, Japanese Patent Laid-open No. 2007-91123 (FIGS. 2 and 6)) incorporated herein by reference.
In FIG. 2 of Patent Document 1, a drive shaft 41 (the reference symbols used in the document are used as they are here) is disposed on the lower side of a pivot shaft 28, a constant-velocity universal joint 82 is connected to the drive shaft 41, and a first final gear 163 (hereinafter referred to as “the pinion gear 163”) meshed with a second final gear 164 provided on the side of a rear wheel 39 is attached to the constant-velocity universal joint 82.
A right end (roller) of the drive shaft 41 is inserted in a cup of the constant-velocity universal joint 82 so as to be movable in the left-right direction in the drawing. It is necessary to fill the cup with oil.
Meanwhile, as shown in FIG. 6 of Patent Document 1, the cup of the constant-velocity universal joint 82 is provided with a hole for attaching the pinion gear 163, a shaft of the pinion gear 163 is passed through the hole, and the pinion gear 163 is fastened to the cup through a nut from the inner side of the cup. Such a structure ensures that the constant-velocity universal joint which is large in size and weight can be disposed as close as possible to a bearing for the pinion gear, so that vibrations due to high-speed rotation of the constant-velocity universal joint can be prevented as securely as possible.
Here, referring to the relationship between the cup and the pinion gear 163, the shaft of the pinion gear 163 is inserted in the through-hole formed in the cup, and the operation of filling the cup with oil must be carried out after the pinion gear and the cup are coupled to each other.
In the above-mentioned structure, in view of the operation of filling the cup with the oil, it is impossible to perform sub-assembly of the constant-velocity universal joint 84 before assembly of the drive shaft 41. Therefore, there is a limitation as to the degree of freedom in assembling the drive shaft, and there is room for improvements from the viewpoint of assuring an assembly work at enhanced speed and efficiency.